Active matrix organic electroluminescent device and fabricating method thereof

ABSTRACT

An organic electroluminescent device including a substrate, a gate line on the substrate, a data line on the substrate, wherein the data line crosses the gate line, a switching element connected to the gate line and the data line, a plurality of driving elements connected to the switching element, each of the plurality of driving elements are interconnected in parallel, and an organic electroluminescent diode connected to each of the plurality of driving elements.

[0001] The present invention claims the benefit of the Korean PatentApplication No. 2002-24553 filed in Korea on May 3, 2002, which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an organic electroluminescentdevice, and more particularly, to an active matrix organicelectroluminescent device including amorphous silicon thin filmtransistor and a fabricating method thereof.

[0004] 2. Discussion of the Related Art

[0005] In general, an organic electroluminescent device (ELD) emitslight by injecting electrons from a cathode and holes from an anode intoan emission layer, combining the electrons with the holes, generating anexciton, and transitioning the exciton from an excited state to a groundstate. Contrary to a liquid crystal display (LCD) device, an additionallight source is not necessary for the organic ELD to emit light becausethe transition of the exciton between states causes light to be emitted.Accordingly, the size and weight of the organic ELD can be reduced. Theorganic ELD has other excellent characteristics such as low powerconsumption, superior brightness, and fast response time. Because ofthese characteristics, the organic ELD is regarded as a promisingcandidate for next-generation consumer electronic applications such ascellular phones, car navigation system (CNS), personal digitalassistants (PDA), camcorders, and palmtop computers. Moreover, sincefabricating the organic ELD is a simple process with few processingsteps, it is much cheaper to produce an organic ELD than an LCD device.

[0006] Two different types of organic ELDs exist: passive matrix andactive matrix. While both the passive matrix organic ELD and the activematrix organic ELD have a simple structure and are formed by a simplefabricating process, the passive matrix organic ELD requires arelatively high amount of power to operate. In addition, the displaysize of a passive matrix organic ELD is limited by its structure.Furthermore, as the number of conductive lines increases, the apertureratio of a passive matrix organic ELD decreases. In contrast, activematrix organic ELDs are highly efficient and can produce a high-qualityimage for a large display with relatively low power.

[0007]FIG. 1 is a schematic cross-sectional view of an organic ELDaccording to the related art. In FIG. 1, an array element 14 including athin film transistor (TFT) “T” is formed on a first substrate 12. Afirst electrode 16, an organic electroluminescent layer 18, and a secondelectrode 20 are formed over the array element 14. The organicelectroluminescent layer 18 may separately display red, green, and bluecolors for each pixel region. Generally, separate organic materials areused to emit light of each color for the organic electroluminescentlayer in each pixel region. An organic ELD is encapsulated by attachingthe first substrate 12 and a second substrate 28, which includes amoisture absorbent material 22, with a sealant 26. The moistureabsorbent material 22 eliminates moisture and oxygen that may penetrateinto a capsule of the organic electroluminescent layer 18. After etchinga portion of the second substrate 28, the etched portion is filled withthe moisture absorbent material 22 and the filled moisture absorbentmaterial is fixed by a holding element 25.

[0008]FIG. 2 is an equivalent circuit diagram of an organicelectroluminescent device according to the related art. In FIG. 2, agate line 14 crosses a data line 16, and a switching element “T_(S)” isconnected to the gate line 14 and the data line 16 at the crossing pointof the gate line 14 and the data line 16. A driving element “T_(D)” iselectrically connected to the switching element “T_(S)” and an organicelectroluminescent diode “D_(EL).” A storage capacitor “C_(ST)” isformed between a driving gate electrode 20 and a driving drain electrodeof the driving element “T_(D),” and the organic electroluminescent diode“D_(EL)” is connected to a power line 22.

[0009] When a scan signal of the gate line 14 is applied to a switchinggate electrode 18 of the switching element “T_(S),” an image signal ofthe data line 16 is applied to the driving gate electrode 20 of thedriving element “T_(D)” through the switching element “T_(S).” Thecurrent density of the driving element “T_(D)” is modulated by the imagesignal applied to the driving gate electrode 20. As a result, theorganic electroluminescent diode “D_(EL)” can display images with grayscale. Moreover, since the image signal stored in the storage capacitor“C_(ST)” is applied to the driving gate electrode 20, the currentdensity flowing into the organic electroluminescent diode “D_(EL)” iskept uniform until the next image signal is applied even when theswitching element “T_(S)” is turned OFF. The switching element “T_(S)”and the driving element “T_(D)” can be formed of a polycrystallinesilicon TFT or an amorphous silicon TFT. The process of fabricating anamorphous silicon TFT is simpler than the process for a polycrystallinesilicon TFT. The amorphous silicon TFT should have a larger width tolength ratio (W/L ratio) to drive the organic electroluminescent diode“D_(EL).” As the W/L ratio of the amorphous silicon TFT becomes larger,the current density flowing through the amorphous silicon TFT increases.High current density may cause the amorphous silicon TFT to degrade dueto stress, thereby disadvantageously changing conductive characteristicsof the amorphous silicon TFT. Moreover, the changes in the conductivecharacteristics of the amorphous silicon TFT are exacerbated when adirect current (DC) bias is continuously applied to the driving element“T_(D)” in the organic electroluminescent device. As a result, thedisplay quality of an amorphous silicon TFT is inferior and may resultin residual images being displayed. In addition, the driving element“T_(D)” may sometimes break down due to the stress caused by theincreased current density. When the driving element “T_(D)” is composedof one TFT, a broken TFT would cause a point defect.

SUMMARY OF THE INVENTION

[0010] Accordingly, the present invention is directed to an organicelectroluminescent device and a fabricating method thereof thatsubstantially obviate one or more of the problems due to limitations anddisadvantages of the related art.

[0011] An object of the present invention is to provide an organicelectroluminescent device where a plurality of driving elements areconnected to each other in parallel, and a fabricating method thereof.

[0012] Another object of the present invention is to provide an activematrix organic electroluminescent device with high display quality and afabricating method thereof.

[0013] Additional features and advantages of the invention will be setforth in the description which follows, and in part will be apparentfrom the description, or may be learned by practice of the invention.The objectives and other advantages of the invention will be realizedand attained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

[0014] To achieve these and other advantages and in accordance with thepurpose of the present invention, as embodied and broadly described, anorganic electroluminescent device including a substrate; a gate line onthe substrate, a data line on the substrate, wherein the data linecrosses the gate line, a switching element connected to the gate lineand the data line, a plurality of driving elements connected to theswitching element, each of the plurality of driving elements areinterconnected in parallel, and an organic electroluminescent diodeconnected to each of the plurality of driving elements.

[0015] In another aspect, a method of fabricating an organicelectroluminescent device, including steps of forming a gate line on asubstrate, forming a switching element connected to the gate line,forming a plurality of driving elements connected to the switchingelement, each of the plurality of driving elements are interconnected inparallel, forming a data line, the data line crosses the gate line andis connected to the switching element, and forming an organicelectroluminescent diode connected to each of the plurality of drivingelements.

[0016] In another aspect, a driving system for an organicelectroluminescent device including a gate electrode, a plurality ofactive layers over the gate electrode, a source electrode on each of theplurality of active layers, the source electrode including an unevenshape having protruding portions and receded portions such that theprotruding portions overlap the gate electrode and the receded portionsdo not overlap the gate electrode, and a drain electrode spaced apartfrom the source electrode, wherein the source electrode and the drainelectrode are symmetrical with respect to the gate electrode.

[0017] In another aspect, a driving element for an organicelectroluminescent device including a gate electrode, an active layerover the gate electrode, a source electrode on the active layer, thesource electrode includes an uneven shape having protruding portions andreceded portions such that the protruding portions overlap the gateelectrode and the receded portions do not overlap the gate electrode,and a drain electrode spaced apart from the source electrode, whereinthe source electrode and the drain electrode are symmetrical withrespect to the gate electrode.

[0018] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are intended to provide further explanation of theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description serve to explain theprinciple of the invention. In the drawings:

[0020]FIG. 1 is a schematic cross-sectional view of an organicelectroluminescent device according to the related art;

[0021]FIG. 2 is an equivalent circuit diagram of an organicelectroluminescent device according to the related art;

[0022]FIG. 3 is an equivalent circuit diagram of an exemplary organicelectroluminescent device according to the present invention;

[0023]FIG. 4 is a schematic plan view of an exemplary driving elementfor an organic electroluminescent device according to the presentinvention;

[0024]FIG. 5 is a schematic plan view of another exemplary drivingelement for an organic electroluminescent device according to thepresent invention; and

[0025]FIG. 6 is an equivalent circuit diagram of an exemplary organicelectroluminescent device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

[0027]FIG. 3 is an equivalent circuit diagram of an exemplary organicelectroluminescent device according to the present invention. In FIG. 3,a gate line 102 and a data line 104 may cross each other on a substrate100, and a switching element “T_(S)” may be disposed at the crossingpoint of the gate line 102 and the data line 104. The switching element“T_(S)” may be electrically connected to a driving element “T_(D).” Thedriving element “T_(D)” may include a plurality of thin film transistors(TFTs) “T_(l)” to “T_(N)” connected to each other in parallel. A sourceelectrode 109 of each of the plurality of TFTs “T_(l)” to “T_(N)” may beelectrically connected to a first electrode of an organicelectroluminescent diode “D_(EL),” and a second electrode of the organicelectroluminescent diode “D_(EL)” may be electrically connected to apower line 106. A storage capacitor “C_(ST)” may be electricallyconnected to a driving gate electrode 108 and a driving drain electrode110 of the plurality of TFTs “T_(l)” to “T_(N).”

[0028]FIG. 4 is a schematic plan view of an exemplary driving elementfor an organic electroluminescent device according to the presentinvention. In FIG. 4, a driving element “T_(D)” may include a pluralityof TFTs. For example, the driving element “T_(D)” may include five TFTs,“T₁” to “T₅.” A gate electrode 108 may be formed on a substrate, and aplurality of active layers 112 may be formed over the gate electrode108. Each of the plurality of active layers 112 may be physically andelectrically separated from adjacent active layers 112 by spaces “b.” Atotal number of active layers may be determinative of a total number ofTFTs. A source electrode 109 and a drain electrode 110 may be formed oneach of the plurality of active layers 112. Each of the sourceelectrodes 109 and each of the drain electrodes 110 may be physicallyseparated from each other and may be symmetrical with respect to thegate electrode 108. In addition, each of the source electrodes 109 andeach of the drain electrodes 110 may have an uneven shape including aprotruding portion “X” and a receded portion “V.” The protruding portion“X” corresponding to each active layer 112 may overlap the gateelectrode 108. In contrast, the receded portion “V” corresponding toeach space between adjacent active layers 112 may not overlap the gateelectrode 108. Accordingly, the protruding portion “X” may function asthe source electrode 109 and the drain electrode 110 for each TFT. Ascurrent density to the driving element “T_(D)” increases, the stresscaused by the high current density may be distributed across theplurality of TFTs used to create the driving element “T_(D).” As aresult, the characteristics of the driving element “T_(D)” may remainsubstantially consistent during operation.

[0029] Since the plurality of active layers 112 are spaced apart fromeach other, a gap “c” between the adjacent protruding portions “X”includes the space “b” between the adjacent active layers 112. Thus, asthe total number of TFTs increases, the area occupied by the drivingelement “T_(D)” increases in proportion to the size of each space “b”between adjacent active layers 112. Moreover, since each of the sourceelectrode 109 and the drain electrode 110 have steps between edges ofthe plurality of active layers 112, an etching solution may permeateinto the receded portions “V” of the source electrode 109 and the drainelectrode 110 due to capillary action. As the etching solution permeatesinto the receded portions “V,” an open between the source electrode 109and the drain electrode 110 may be created.

[0030]FIG. 5 is a schematic plan view of another exemplary drivingelement for an organic electroluminescent device according to thepresent invention. In FIG. 5, an active layer 112 may be formed over agate electrode 108, and a source electrode 109 and a drain electrode 110may be formed on the active layer 112. The source electrode 109 and thedrain electrode 110 may be physically separated from each other and maybe symmetrical with respect to the gate electrode 108. The sourceelectrode 109 and the drain electrode 110 may each have an uneven shapeincluding a protruding portion “X” and a receded portion “V,” whereinthe protruding portion “X” may overlap the gate electrode 108, and thereceded portion “V” may not overlap the gate electrode 108. A drivingelement “T_(D)” may include a plurality of TFTs. For example, thedriving element “T_(D)” may include five TFTs, “T₁” to “T₅.” Theplurality of TFTs may be formed between the protruding portion “X” ofthe source electrode 109 and the drain electrode 110. Accordingly, sincethe active layer 112 is unified, the driving element “T_(D)” may requireless area. Moreover, since the source electrode 109 and the drainelectrode 110 have fewer steps, the potential for an open between thesource electrode 109 and the drain electrode 110 may be reduced.Furthermore, as current density to the driving element “T_(D)”increases, the stress caused by the high current density may bedistributed across the plurality of TFTs used to create the drivingelement “T_(D).” As a result, the characteristics of the driving element“T_(D)” may remain substantially consistent during operation.

[0031] The storage capacitor “C_(ST)” (in FIG. 3) may be electricallyconnected to the driving gate electrode 108 and the driving drainelectrode 110. When the driving element “T_(D)” possesses the structureshown in FIG. 5, the protruding portion “X” of the drain electrode 110may overlap the gate electrode 108. Since the overlapping portion of thegate electrode 108 and the protruding portion “X” functions as a storagecapacitor, an extra storage capacitor may not be necessary.

[0032]FIG. 6 is an equivalent circuit diagram of another exemplaryorganic electroluminescent device according to the present invention. InFIG. 6, a storage capacitor may not be included between a driving gateelectrode 108 and a driving drain electrode 110 of a driving element“T_(D),” in contrast with the exemplary circuit diagram of FIG. 3.

[0033] The organic electroluminescent device using amorphous siliconthin film transistors as switching and driving elements includes aplurality of thin film transistors as the driving element. Since theplurality of thin film transistors are connected in parallel to eachother, stress caused by high current density is distributed between theplurality of thin film transistors. Therefore, characteristics of theplurality of thin film transistors remain substantially consistentduring operation. As a result, a high display quality may be obtained.

[0034] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the organicelectroluminescent device and fabricating method thereof of the presentinvention without departing from the spirit or scope of the invention.Thus, it is intended that the present invention cover the modificationsand variations of this invention provided they come within the scope ofthe appended claims and their equivalents.

What is claimed is:
 1. An organic electroluminescent device, comprising:a substrate; a gate line on the substrate; a data line on the substrate,wherein the data line crosses the gate line; a switching elementconnected to the gate line and the data line; a plurality of drivingelements connected to the switching element, each of the plurality ofdriving elements are interconnected in parallel; and an organicelectroluminescent diode connected to each of the plurality of drivingelements.
 2. The device according to claim 1, wherein the plurality ofdriving elements include amorphous silicon thin film transistors.
 3. Thedevice according to claim 2, further comprising a storage capacitorconnected in parallel to the plurality of driving elements.
 4. Thedevice according to claim 3, wherein the switching element includes anamorphous silicon thin film transistor.
 5. The device according to claim1, further comprising a power line parallel to the data line andelectrically connected to each of the plurality of driving elements. 6.A method of fabricating an organic electroluminescent device, comprisingsteps of: forming a gate line on a substrate; forming a switchingelement connected to the gate line; forming a plurality of drivingelements connected to the switching element, each of the plurality ofdriving elements are interconnected in parallel; forming a data line,the data line crosses the gate line and is connected to the switchingelement; and forming an organic electroluminescent diode connected toeach of the plurality of driving elements.
 7. The method according toclaim 6, wherein the plurality of driving elements include amorphoussilicon thin film transistors.
 8. The method according to claim 7,further comprising a step of forming a storage capacitor connected inparallel to the plurality of driving elements.
 9. The method accordingto claim 8, wherein the switching element includes an amorphous siliconthin film transistor.
 10. The method according to claim 6, furthercomprising a step of forming a power line parallel to the data line andelectrically connected to each of the plurality of driving elements. 11.A driving system for an organic electroluminescent device, comprising: agate electrode; a plurality of active layers over the gate electrode; asource electrode on each of the plurality of active layers, the sourceelectrode including an uneven shape having protruding portions andreceded portions such that the protruding portions overlap the gateelectrode and the receded portions do not overlap the gate electrode;and a drain electrode spaced apart from the source electrode, whereinthe source electrode and the drain electrode are symmetrical withrespect to the gate electrode.
 12. The driving system according to claim11, wherein the plurality of active layers include amorphous silicon.13. The driving system according to claim 11, wherein the protrudingportions overlap each of the active layers.
 14. The driving systemaccording to claim 11, wherein each of the plurality of active layers isphysically separated from adjacent active layers.
 15. The driving systemaccording to claim 14, wherein the receded portions overlap spacesbetween adjacent active layers.
 16. A driving element for an organicelectroluminescent device, comprising: a gate electrode; an active layerover the gate electrode; a source electrode on the active layer, thesource electrode includes an uneven shape having protruding portions andreceded portions such that the protruding portions overlap the gateelectrode and the receded portions do not overlap the gate electrode;and a drain electrode spaced apart from the source electrode, whereinthe source electrode and the drain electrode are symmetrical withrespect to the gate electrode.
 17. The driving element according toclaim 16, wherein the active layer includes amorphous silicon.
 18. Thedriving element according to claim 16, wherein the protruding portionsoverlap the active layer.